1. Field of the Invention
The present invention relates to a capacitor element for a solid electrolytic capacitor such as tantalum capacitor or aluminum capacitor. The present invention also relates to a device and a process for making such a capacitor element.
2. Description of the Related Art
As disclosed in U.S. Pat. No. 5,461,538 (corresponding to Japanese Patent Application Laid-open No. 7-74062) and as illustrated in FIG. 9 of the accompanying drawings, a typical capacitor element for a prior art solid electrolytic capacitor includes a porous capacitor chip A1 and an anode wire A2 projecting from a top face A1a of the chip A1. The capacitor chip A1 is prepared by compacting tantalum powder into a porous mass and then sintering the porous mass. The anode wire A2 is also made of tantalum. For providing a capacitor function, the capacitor element is subjected to the following process steps.
First, as shown in FIG. 10, the porous sintered capacitor chip A1 is immersed in an aqueous solution B of e.g. phosphoric acid and subjected to anodic oxidation (electrolytic oxidation) by applying a direct current. As a result, a dielectric coating A3 of e.g. tantalum pentoxide is formed on the surfaces of the tantalum particles and on an immersed root portion of the anode wire A2, as shown in FIG. 11.
Then, the dielectrically coated chip A1 is immersed in an aqueous solution C of e.g. manganese nitrate to such an extent that the top surf ace A1a of the chip A1 is not submerged under the surface of the manganese nitrate solution, the chip A1 being thereafter taken out of the solution for baking. This step is repeated plural times to form a layer A4 of solid electrolyte (e.g. manganese dioxide) on the dielectric coating A3.
Finally, a metallic cathode terminal layer (made of silver or nickel for example) is formed on the solid electrolyte layer with an intervening layer of e.g. graphite being interposed between the cathode terminal layer and the electrolyte layer A4.
A solid electrolytic capacitor incorporating such a capacitor element is known to exhibit a considerably higher impedance under high frequency than a lamination type capacitor. The impedance of the solid electrolytic capacitor is inversely proportional to the contact surface area between the dielectric coating A3 and the solid electrolyte layer A4.
For purposes of decreasing the impedance of a solid electrolytic capacitor, a capacitor element is proposed, in U.S. Pat. No. 3,345,545 for example, which includes a porous capacitor chip A1' of tantalum powder laterally formed with a plurality of grooves A6', and an anode wire A2' projecting from the top face A1a' of the chip A1', as shown in FIGS. 14 and 15 of the accompanying drawings. Each of the grooves A6' extends all the way from the top face A1a' to the bottom face A1b'.
The grooves A6' on the capacitor chip A1' increase the contact surface area between a dielectric coating (formed subsequently) and a solid electrolyte layer (also formed subsequently), thereby decreasing the impedance of the capacitor. Further, the grooves A6' also facilitate permeation of a manganese nitrate solution into the porous capacitor chip A1' at the time of forming the solid electrolyte layer. However, the capacitor element shown in FIGS. 14 and 15 has been found still disadvantageous in the following points.
First, since each of the grooves A6' extends all the way from the top surface A1a' of the chip A1' to the bottom surface A1b', the electrostatic capacity of the capacitor element reduces considerably due to the provision of the grooves. Thus, the size of the capacitor element need be increased in an axial direction of the anode wire A2' to compensate for a volumetric reduction resulting from the provision of the grooves A6'.
Secondly, since each of the grooves A6' extends all the way from the top surface A1a' of the chip A1' to the bottom surface A1b', a manganese nitrate solution is allowed to flow down quickly along the grooves A6' when the chip A1' is taken out of the solution at the time of forming a solid electrolyte layer. Thus, the process steps of immersing the chip A1' in the manganese nitrate solution and thereafter baking the chip need be repeated sufficiently until the solid electrolyte layer grows completely.